About Cobalt-60 & Gamma Irradiation
Cobalt-60 starts out as Cobalt 59 which is a naturally occurring metal. This metal is mined, typically as a by-product of copper and nickel mining. High-purity Cobalt-59 powder is compressed and formed into solid slugs or pellets that are nickel plated and then sealed into metal tubes. These tubes, called targets, are placed in a nuclear reactor where the Cobalt-59 absorbs neutrons from the nuclear reaction process and becomes Cobalt-60.
Gamma radiation is part of the electromagnetic energy spectrum like microwaves, radio waves, infrared radiation, ultraviolet light, X-rays and visible light.
Gamma rays are ionizing radiation which means that they carry enough energy to liberate an electron from an atom or molecule, ionizing it. Therefore, gamma radiation will cause damage to the DNA of living organisms and will change the molecular structure of certain materials.
Cobalt-59 is typically placed in a reactor for 1-2 years to produce Cobalt-60 for industrial applications. After this period the targets are removed from the nuclear reactor and transferred to a facility where the Cobalt-60 is removed and encapsulated in stainless steel. These stainless steel capsules, usually called Cobalt-60 sources or pencils, are of tested and approved industry standard designs for use in gamma processing.
The Cobalt-60 sources are then supplied to gamma processing facilities that use them as a source of gamma radiation.
Radiation processing, also called gamma irradiation, is the use of gamma radiation on an industrial scale to treat a range of products. In a gamma processing facility, Cobalt-60 sources are positioned in a rack that is located inside a concrete bunker called the irradiation cell. Product to be treated is carried into the irradiation cell by way of a conveyor system and circulated around the Cobalt-60 sources until it has received the required dose of radiation. A process called dosimetry is used to check that product has received the correct dose of radiation. It is important that enough radiation is received by the product to achieve the desired effect without giving too much dose that may damage the product.
When the Cobalt-60 sources are not in use, the rack is lowered into its safe storage pool as shown in the photo. The radiation gives off a blue glow underwater, known as the Cherenkov Effect, named after the Russian scientist who studied the phenomenon. This is caused by charged particles travelling faster than the speed that light normally travels through water.
Photograph courtesy of Nordion (Canada) Inc.
The largest volume of product treated by gamma processing is single use disposable medical devices such as surgical gowns, gloves, syringes, catheters, blood-bags and dressings. This type of product is treated with radiation to kill bacteria and sterilize the product so that it can be safely used in operations and other healthcare applications. Gamma processing is a highly effective sterilization method because the radiation penetrates the product packaging and the internal and external surfaces of the product itself. Therefore, complex products such as theatre packs can be sterilized inside their final packaging and will be ready for use immediately after processing.
Food is also treated with gamma irradiation to kill harmful micro-organisms that may lead to food borne illness and food spoilage. Herbs and spices are treated to ensure that harmful pests and bacteria do not enter our food with these ingredients; meat, poultry and sea food products are treated to eliminate pathogens such as e. coli and salmonella; and tropical fruit is treated to slow down spoilage, stop sprouting and delay ripening.
Other applications include the treatment of pharmaceutical and cosmetics raw material that may contain high levels of bacteria; the irradiation of empty food packaging to control mircobial contamination; and the treatment of materials to improve their physical properties such as strength and resistance to high temperature.
Electron beam and x-ray irradiation is also used in radiation processing. These technologies use machine generated electrons or x-rays to give the required radiation dose for similar applications.